It is known that organic optoelectronic devices such as OLEDs, devices comprising organic photovoltaic cells and those comprising organic TFTs, need to be encapsulated to ensure that their sensitive components are protected from the gaseous species of the atmosphere (mainly oxygen and water vapor). This is because, if suitable protection is not provided, there is a risk that the device will subsequently degrade, this degradation exhibiting itself mainly by the appearance of non-emissive black spots in the case of OLEDs, which spots are in fact the result of water vapor penetrating into the diode and degrading the cathode (or anode)/organic film(s) interface.
This encapsulation can typically be achieved using a glass cap adhesively bonded to the organic device using a specific adhesive especially having a low water permeability. In general a solid moisture getter is added between the substrate and the cap in order to increase the lifetime of the device.
For certain applications, but also to reduce cost, thin films have been developed that act as barriers and the role of which is comparable to the cap/getter assembly, i.e. they protect the subjacent device from attack by moisture. Generally, these barrier films are oxides, nitrides or oxynitrides or, in certain cases, they may be thin metal films unless the light-emitting unit is a top-emitting structure in which case the barrier films must be transparent.
These thin films are deposited by standard vacuum deposition processes such as chemical vapor deposition (CVD), which is optionally plasma enhanced (PECVD), atomic layer deposition (ALD, sometimes called ALCVD) or by physical vapor deposition (PVD) processes including evaporation and sputtering. To produce a barrier, CVD and in particular ALD technologies are especially preferred because, being low-temperature technologies, they provide, at temperatures most often below 110° C., barrier films that are dense, have few pinholes, that are 100% conformal and that are compatible with OLEDs. Thus a film of Al2O3 deposited at a low temperature by ALD has been obtained with a pinhole density as low as 38 /cm2. For microdisplay applications, this pinhole density is nevertheless too high because, if a microdisplay with an area of 45 mm2 is considered, the above density leads to 17 pinholes per microdisplay, i.e. potentially 17 dark spots on the OLED display. Indeed, even though these dark spots intrinsic to the fabrication process are microscopic, their presence is unacceptable in an OLED device the image of which is magnified by suitable optics, and additional “extrinsic” dark spots due to the presence of undesirable particles on the surface of the device during the thin-film encapsulation must furthermore be taken into account.
Moreover, it is known that it is necessary to provide long-term protection of Al2O3 deposited by ALD from water because it has a tendency to hydrolyze into Al(OH)x. It has therefore been sought to durably passivate the barrier that such an Al2O3 film forms using more chemically inert and more stable inorganic materials such as SiO2, Si3N4 or SiOxNy deposited by low-temperature PECVD, which passivation also allows residual pinholes in these Al2O3 films to be filled.
As a variant, it has been sought to passivate these ALD-deposited Al2O3 films with relatively thick organic films based on “planarizing” polymers that are supposed to alleviate the drawback of the aforementioned undesirable particles by coating them with a multilayer of alternating organic and inorganic films, such as the Barix™ multilayer from Vitex. One drawback of this solution lies in the gaseous-phase “flash” evaporation process (i.e. evaporation of the monomer, condensation on the substrate, then UV curing) used since it is relatively costly in terms of time.
It is difficult to envision using other deposition techniques that are less costly in terms of time such as liquid-phase deposition because liquid-phase deposition requires polymer solutions containing solvents that are liable to dissolve the films of the subjacent light-emitting unit.